Biology · Animal Kingdom · NEET
Symmetry describes how you can cut an animal's body with an imaginary flat sheet (called a plane) and get two matching halves. If some plane splits the body into two equal, mirror-image parts, the animal has symmetry. If no plane can do this, the animal is asymmetrical. NCERT uses body symmetry as one of the fundamental features to classify the whole animal kingdom, so it is a starting point for the whole chapter.
Sponges (Phylum Porifera) are mostly ASYMMETRICAL. NCERT says clearly: any plane that passes through the centre does NOT divide a sponge into two equal halves. So there is no line of symmetry. A common mistake is to guess radial because sponges look tube-like, but the textbook answer is asymmetrical. Only a very few sponges show radial symmetry, so for NEET remember: Porifera = asymmetrical.
In RADIAL symmetry, ANY plane passing through the central axis divides the body into two identical halves, so there are many planes of symmetry (think of a wheel or a cake). In BILATERAL symmetry, ONLY ONE plane divides the body into identical left and right halves, so there is just one plane of symmetry (think of your own body). Radial animals have a top and bottom but no left/right or front/back; bilateral animals have a clear head end (anterior) and tail end (posterior).
NCERT names three groups: Coelenterates (Cnidaria, like Hydra and jellyfish), Ctenophores (comb jellies), and Echinoderms (like starfish) — but only the ADULT echinoderm is radial. These animals are usually sessile (fixed) or slow floating, so a body that is the same on all sides helps them sense food and danger coming from any direction.
Both, but at different life stages. This is the most tested trap in this concept. The ADULT echinoderm (like Asterias, the starfish) is RADIALLY symmetrical, but its LARVA is BILATERALLY symmetrical. So if a question says 'adult echinoderm is bilateral', it is WRONG, and if it says 'echinoderm larva is bilateral', it is RIGHT. Remember: Echinoderm = radial adult, bilateral baby.
Animals like annelids, arthropods, molluscs, chordates and humans move forward with a definite head. Bilateral symmetry gives one clear front end where sense organs and the brain gather (this is called cephalisation), which helps directed movement. That is why active, moving animals are bilateral, while fixed or drifting animals tend to be radial.
Asymmetrical = ZERO planes of symmetry (no equal halves possible). Radial = MANY planes (any plane through the central axis works). Bilateral = exactly ONE plane (only the plane down the middle giving left and right). NEET sometimes hides the definition inside a statement question, so knowing 'one plane vs any plane' lets you answer instantly.
Statement I: When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry. Statement II: In phylum Echinodermata, both adults and larvae are radially symmetrical. Choose the most appropriate answer:
Radial symmetry is NOT found in adults of phylum _______.
Bilaterally symmetrical and acoelomate animals are exemplified by
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Asymmetrical (no plane gives equal halves, e.g. sponges), radial (any plane through the central axis gives equal halves, e.g. Cnidaria, Ctenophora, adult echinoderms), and bilateral (only one plane gives left and right equal halves, e.g. annelids, arthropods, humans).
Porifera (sponges) are mostly asymmetrical, according to NCERT. No plane passing through the centre divides them into two equal halves.
Ctenophora (comb jellies) are radially symmetrical, along with Coelenterata and adult Echinodermata.
Because the adult and larva differ: the adult echinoderm is radial but the larva is bilateral. NEET repeatedly tests whether students remember that the LARVA is bilateral, not the adult.
Use A-R-B = Absent (asymmetrical), Round (radial, any plane), Both-sides (bilateral, one left-right plane). For echinoderms: radial adult, bilateral baby.
